Although thrombolytic therapy for acute stroke continues to gain wider acceptance and usage, there remains a compelling need for neuroprotective therapy and new treatment to reduce thrombolytic complications. Thrombin is a serine protease that plays a critical role in the coagulation cascade. Thrombin induces protection at low doses (thrombin preconditioning) but acts as a neurotoxin at high doses, killing cells via the protease activated receptors (PARs). We now propose to test the hypothesis that thrombin partially mediates edema and cell death during stroke via the PAR-1 receptor signaling pathway. Using a protease-cleavable cell-penetrating probe developed by Dr. Roger Tsien, we will determine directly whether thrombin activation mediates edema and tissue injury. Using a highly reproducible and quantifiable model of ischemic edema and tissue injury, we will test a variety of pharmacological manipulations of coagulation or PAR-1 and its presumed signaling pathways. Using lentiviral mediated RNA interference, we will reduce gene expression of PAR-1 in focal areas of the parietal cortex of adult mice prior to standard MCAo and then determine the effect of PAR-1 knock-down on tissue injury. Using knock-out mice deficient in PAR-1, PAR-2, PAR-3, or PAR-4 subjected to MCAo, we will determine whether each or all PAR receptors influence vascular disruption and tissue injury after MCAo. Finally, we will extend our previous studies to include behavioral measures of cerebral injury and seek to determine whether thrombin exacerbates-and whether argatroban ameliorates-behavioral deficits after MCAo. We will use intra-arterial mutant thrombin designed to activate PAR-1 receptor but not affect coagulation during ischemia. We will also test neuroprotective drugs such as mutant APC or others screened for effect on stroke outcome. Taken together, these studies will reveal whether thrombin cytotoxicity plays a role in stroke and whether agents active at PAR-1 signaling pathway represent an effective therapy. Relevance The search for effective neuroprotectants is hampered by many factors: we continue to lack a full understanding of the molecular mechanisms of ischemic cell death and vascular disruption, as well as the patho-anatomic mechanisms of ischemic edema, infarction, and behavioral impairment. Fundamental exploration of molecular and patho- anatomic mechanisms AND translational studies of therapeutics remain highly significant and critical to understanding and treating stroke, the most common cause of adult disability in the world. PUBLIC HEALTH RELEVANCE: Stroke, the most common cause of adult disability in the world, is the third leading cause of death in America, yet we lack effective therapy for most patients. This proposal seeks to find new, powerful treatments for the most common cause of stroke, and to elucidate some of the fundamental mechanisms underlying brain ischemia by studying a common and important blood protein, thrombin. The proposal will enable us to confirm and extend our promising preliminary findings as well as help us understand how thrombin contributes to brain injury after stroke.